Crystal Oscillators

You will often find these used with microprocessors & microcontrollers.

Its an electronic oscillator circuit that uses the mechanical resonance of a vibrating crystal of piezoelectric material to create an electrical signal with a precise frequency.
(Wikipedia)
They are a type of linear oscillator made of quartz.

This is a 16 Hz quartz crystal. It uses a HC-49/S package.

This is also a 16Mhz crystal . The package is a HC49/U


These oscillators are not to be confused with your usual analog VCOs.
The crystal oscillator provides a fixed frequency. They often are the clock for a microprocessor, UART, etc. 

Through Hole Crystal (THD)Package types vary.
They include HC49, HC50, UM, AT26, AT39

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 Model                         Frequency Range Mode of operation

HC49/US                     3.0 MHz ~ 54.0 MHz Fundamental 36.0 MHz ~ 150.0 MHz 3rd-overtone

HC49/U

HC-49/S package.

UM-1 UM-5 UM-4    8.0 MHz ~ 70.0 MHz Fundamental 35.0 MHz ~ 200.0 MHz 3rd-overtone

This is a UM-1 low pass filter

A UM-5

AT26                            6.0 MHz ~ 27.0 MHz Fundamental
The AT packages are cylindrical in shape.

THey of course also come in SMD packages.

This is a ABLS7M2-12.288MHZ-D-2Y-T - 
Crystal, 12.288 MHz, SMD,
7mm x 4.1mm, 30 ppm, 18 pF, 20 ppm, ABLS7M2 Series